2. • In Organic Chemistry, a Witting reaction is used to prepare an alkene
by reacting an aldehyde or ketone with an ylide. An ylide is a
compound that has two bound atoms that are oppositely charged.
• This compound is known as an organo-phosphorous or phosphonium
salt. Because of phosphorus's chemical prosperities, the
phosphonium salt has two charged particles bound together and a
resonance structure can form (fig. 1), making a reaction possible.
3. • The Wittig synthesis reaction is preferable over elimination reactions
when the regional-selectiveness of the double bond is desired for
higher yields. The mechanism replaces the carbonyl’s double bonded
O with a double bonded C without producing constitutional isomers.
• The Wittig reaction is named after the German scientist, Georg Wittig,
who discovered it and in 1979 he was awarded a Nobel Prize.
4. • The three step mechanism of the Wittig reaction is:
• The negatively charged C atom of the ylide acts as the nucleophile, attacking
the partial positive charged C atom of a carbonyl. This displaces the π
electrons to the O atom.
• The lone pair then re-bonds to the P atom of the salt.
• The thermodynamic drive of the intermediate continues the reaction forward
in a single step, reducing the carbonyl to an alkene while oxidizing the salt.
5. • Two examples of the Wittig reaction can be experimentally achieved
by the synthesis of:
4-Methoxybenzealdehyde to
4-Methoxystyrene (Fig. 2)
2,4-Dimethoxybenzealdehyde to
2,4-Dimethoxystyrene (Fig. 3)
6. • 4-Methoxybenzealdehyde 4-Methoxystyrene
• 2,4-Dimethoxybenzealdehyde 2,4-Dimethoxystyrene
2.04 g 4-Methoxybenzealdehyde
136.148 M 4-Methoxybenzealdehyde
Theoretical: (134.178 4-Methoxystyrene) = 2.01g
2.49 g 2,4-Methoxybenzealdehyde
154.167 M 2,4-Methoxybenzealdehyde
Theoretical: (152.193 2,4-Methoxystyrene) = 2.01g
1.7 g 4-Methoxystyrene
2.01g
Actual: (100%) = 84.6%
1.94 g 2,4-Methoxystyrene
2.458g
Actual: (100%) = 78.9%
7. There are slight differences between the
spectrum of the two products.
Can you guess why?
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Synthesis of the products can be evaluated using 1H NMR.
12. The extra Methoxy group gives a
different spectrum reading because
of the shielding effect from the
donation of electrons from the
oxygen.
In these NMRs, there is a
shift in the Benzene peaks
as well as distortion
from the different hydrogens.
There are slight differences between the spectrum of the two products.
